A Photonic Crystal (PC) is a microstructured material with wavelength- and angle-dependent optical properties. For a Photonic Band Gap (PBG) to exist within a PC, the allowed quantum energy bands must not overlap for some area, and that gap area is the PBG. The PBG may exist in one, two, or three dimensions and in TE and/or TM modes. A complete 3-D PBG is a gap in the allowed quantum states across all propagation directions and polarization modes. The band gap position and gap width may be easily modified to yield desired photonic properties by varying crystal parameters, such as: structural geometry, crystal lattice dimensions, or contrast of indices of refraction between the composite material and vacuum/air. PBG properties are generally fully established within only a few lattice constants. Because of these properties, PBGs are attractive for sharp filters and lossless waveguides in both telecommunication and optical computing applications. Since certain wavelengths cannot propagate, the PBGs' optical emissions are centered in the band of interest and not spread over a wide spectrum as with a blackbody. Therefore, thermal stimulation of PBGs can benefit lighting and ThermoPhotoVoltaic (TPV) applications. An in-depth overview of PCs, PBGs, and fabrication methods can be found at: Cefe Lopez, “Materials Aspects of Photonic Crystals”, Advanced Materials 15, p1679 (2003) or http://ab-initio.mit.edu/photons/tutorial/.
A complete 3-D PBG may exist in a variety of PC structures. One such structure is a stacked array of rods, known as a woodpile structure. Woodpile structures are fabricated using nanolithographic techniques modified from well-known semiconductor processes. These structures must be painstakingly assembled one layer at a time. For each layer, one must create a pattern, etch a mould, fill a mould, and polish that layer into a plane. In addition, for visible emission applications, feature size is on the order of 100 nm, requiring state-of-the-art lithography and exceptional layer-to-layer registration quality control. This method is too costly in terms of time, capital, and material to be suitable for most applications. Further reference is available at U.S. Pat. No. 6,358,854 B1 “Method to fabricate layered material compositions”.
Another complete 3-D PBG structure is an inverse opal. An inverse opal is the volumetric inverse of an opal. An opal, which is a PC structure without a PBG, is a closely packed array of uniformly sized spheres. An inverse opal is built by using an opal as a form and filling the residual (interstitial) volume between the spheres themselves with a material whose refractive index contrast to vacuum/air is high. For visible or near-IR light emission applications, the lattice size required is in the range of 400 nm to 1 μm. Unfortunately, there are currently few suitable materials within this size range. Molecular-based templates are too small, while traditional mechanical manipulations are too large, although they can be useful in microwave applications. Even though natural opal gemstones are of about the right particle diameter, they are impractical due to: too small of crystal domain size, particle size variations, limited availability, and extremely high cost.
Since there are limited templates available that are suitable for micromoulding, one must be synthesized. One method to synthesize an opal structure is via a controlled withdrawal process: taking a colloidal suspension of spheres (typically silica or polystyrene), inserting a substrate into the suspension in order to create a meniscus line, and then slowly evaporating the suspending agent (typically water). The surface tension of the evaporating water at the top of the meniscus line pulls the spheres into a closely packed array no more than a few layers thick, leaving an opal structure of spheres. This is a slow process, taking days to months to grow a sample of commercially useful size and quality. Faster withdrawal of the substrate results in faster growth rates but at the expense of more defects. Another synthesis process includes ultra-centrifuging a suspension of spheres to produce an opal sediment. The centrifugal force packs the spheres into a closely packed array at the bottom of the centrifuge tube. As a result, the sediment is very thick and does not cover a large surface area. Since PBG properties are fully developed in only a few lattice constants, thick structures not only cost more due to additional material but also offer no benefit since most applications desire a thin sediment over a relatively larger surface area. Yet another opal synthesis process includes a self-assembly sedimentation process using electrophoresis. A suspension of surface-charged spheres is placed between two plates, and as a voltage is applied between the two plates, the charged spheres are attracted toward the oppositely charged electrode, thereby modifying the sedimentation rate. If the resultant sedimentation rate is too slow, an opal sediment will still be formed, but very slowly. If the resultant sedimentation rate is fast, the sedimentation time will be decreased but at the expense of increased dislocation defects and smaller domain size. If the resultant sedimentation rate is even faster, the sediment will be a random sludge. This process is further described in US 2003/0,156,319 A1 “Photonic Bandgap Materials Based on Silicon”. In all of the above opal structure fabrication methods, high quality samples take a very long time to grow; however, lower quality samples can be grown faster. Large samples are on the order of only a few square centimeters. Another limitation is that the opals are very fragile, easily damaged by handling and by shrinkage during drying. The opal structures must be dried before they are inverted. Necking adds structural stability. Silica opals are necked by drying the opal structure and then sintering, slightly melting the spheres such that they partly fuse together. Necking has the added benefit of subsequently optimizing the width of the band gap. Therefore, a system and method to fabricate opal structures quickly, with low defects, large domain size, and at low cost is needed.
An inverse opal structure can be formed from a synthesized opal structure in several ways. One such method starts with a self-assembled opal structure made with silica spheres. Then, a Chemical Vapor Deposition (CVD) of Si, Ge, or other metal is applied in the interstitial spaces between the spheres, and the silica spheres are then etched out with a hydrofluoric acid solution. This process does not completely fill the interstitial spaces, creating only an optically thin layer of material or ‘eggshell’ around the spheres. Thicker shells are possible, but require notably longer deposition times. Thin materials are fragile and are not thermally conductive. Other limitations include: the use of toxic CVD gasses and hydrofluoric acid as well as the need for vacuum during processing. This example selection of inverse opal materials exhibits a complete 3-D PBG. Further reference is found at “Self-assembly lights up”; Nature; Nov. 15, 2001.
Another inverse opal structure is formed around a polystyrene opal structure. An electroplating electrolyte is infiltrated into the interstitial spaces between the spheres; a CdSe alloy is electro-deposited; and, subsequently, the spheres are removed by dissolution in toluene. The result is a CdSe inverse opal PBG crystal. One disadvantage is the risk of damage to the opal structure when the electrolyte is infiltrated. Further reference is made in “Electrochemically grown photonic crystals”; Nature; Dec. 9, 1999. Therefore, a system and method for high quality, low-cost inverse opal structures and inverse opal PBG crystals is needed.
Thermal stimulation of a PBG device benefits applications including: lighting, TPV power generation, and thermal signature modification. Since PBG devices modify the available quantum mechanical states within the structure, when thermally stimulated to around 1000° C., they do not behave as black bodies but have a more narrowband optical emission. The quantum mechanical form of Plank's law still applies. As with any thermal stimulation, the radiated energy is not coherent. Attempts to thermally stimulate existing PBG structures have limitations. A tungsten woodpile structure has been thermally stimulated in U.S. Pat. No. 6,583,350 “Thermophotovoltaic energy conversion using photonic bandgap selective emitters”. However, fabrication of a woodpile structure is too costly to be commercially accepted in a commodity market. An ‘eggshell’ CVD inverse opal structure suffers from a high thermal resistance, making it more difficult to thermally stimulate. This structure also lacks a substrate, which is required in direct heating applications. The electro-deposited CdSe inverse opal structure will melt at high temperature and will evaporate in high vacuum. Therefore, a system and method for a high temperature, low vapor pressure, high quality, low-cost inverse opal PBG crystals is needed.